Propulsion Airframe Integration Research Articles

A Review of Novel and Non-Conventional Propulsion Integrations for Next-Generation Aircraft

The aim of this review paper is to collect and discuss the most relevant and updated contributions in the literature regarding studies on new or non-conventional technologies for propulsion–airframe integration. Specifically, the focus is given to both evolutionary technologies, such as ultra-high bypass ratio turbofan engines, and breakthrough propulsive concepts, represented in this frame by boundary layer ingestion engines and distributed propulsion architectures. The discussion focuses mainly on the integration effects of these propulsion technologies, with the aim of defining performance interactions with the overall aircraft, in terms of aerodynamic, propulsive, operating and mission performance. Hence, this work aims to analyse these technologies from a general perspective, related to the effects they have on overall aircraft design and performance, primarily considering the fuel consumption as a main metric. Potential advantages but also possible drawbacks or detected showstoppers are proposed and discussed with the aim of providing as broad a framework as possible for the aircraft design development roadmap for these emerging propulsive technologies.

Propulsion–Airframe Integration for Conceptual Redesign of a Low-Boom Supersonic Transport

A low-boom supersonic transport was generated in a previous low-boom multidisciplinary optimization study that used computational fluid dynamics (CFD) off-body pressure to compute the undertrack sonic boom ground signature and calibrated low-fidelity aerodynamic analyses to compute the mission performance metrics. This low-boom aircraft, referred to as the Mach 1.7 40-PAX concept, can carry 40 passengers for a low-boom overland mission with cruise Mach 1.7 for the airport pairs over the continental United States and has the potential to achieve an undertrack sonic boom ground noise level below 70 perceived level of decibels at the start of overland cruise (SOC). The engine for the Mach 1.7 40-PAX concept was designed using the Numerical Propulsion System Simulation (NPSS) for the mission analysis and modeled as a flow-through nacelle for CFD-based sonic boom analysis. To understand how the engine plume affects the undertrack ground signature, the Mach 1.7 40-PAX concept is redesigned in this paper after replacing the flow-through nacelles with CFD engines for sonic boom analysis using aeropropulsive CFD simulation. A process is developed for approximation of the NPSS engine at SOC by a CFD engine for aeropropulsive CFD simulation. The generated CFD engine has the identical nozzle boundary conditions and approximately the same mass flow and thrust as those of the NPSS engine at SOC. Then, the outer mold line of the configuration with the CFD engines is redesigned to approximately restore the low-boom characteristics of the Mach 1.7 40-PAX concept at SOC. Finally, the CFD simulation data for the redesigned concept with the CFD engines are used to calibrate the low-fidelity aerodynamic analyses for mission analysis of this concept. The cyclic dependency of the involved disciplinary analyses is resolved using an iteration method for a consistent coupling of the mission analysis, NPSS engine analysis, and low-boom redesign using the aeropropulsive CFD simulation. This low-boom redesign study is used as an example to demonstrate how the propulsion–airframe integration could be implemented for conceptual design of supersonic transports that satisfy both the low-boom and mission performance requirements.

Wind tunnel investigation of different engine layouts of a blended-wing-body transport

A 2% scale, cruising version of a 450-seat class Blended-Wing-Body (BWB) transport was tested in the China Aerodynamic Research and Development Center’s FL-26 2.4-by-2.4-meter subsonic wind tunnel. The focus of the wind tunnel test was to investigate the aerodynamic performance of the latest BWB transport design, which would also aid in choosing a final engine arrangement in the three most potential engine integration layouts. The wind tunnel model can be tested with and without the nacelle and has three sets of different nacelle/tail integration positions. Computational Fluid Dynamics (CFD) simulations were performed in engine-aircraft integration design to find appropriate nacelle installing parameters of each layout. The comparison of CFD with experimental results shows good agreement. Wind tunnel measurements indicate that the tail-mounted engine layout produces the minimum drag penalty, while the fuselage-mounted engine layout increases drag the most. Experimental pressure measurement illustrates the effect of nacelle integration on the wing-body surface pressure distribution. This experimental and numerical research provides a reference for future BWB Propulsion-Airframe Integration (PAI) design.

Techno-economic-environmental evaluation of aircraft propulsion electrification: Surrogate-based multi-mission optimal design approach

Driven by the sustainability initiatives in the aviation sector, the emerging technologies of aircraft propulsion electrification have been identified as the promising approach to realize sustainable and decarbonized aviation. This study proposes a surrogate-based multi-mission optimal design approach for aircraft propulsion electrification, which innovatively incorporates realistic aviation operations into the electric aircraft design, with the aim of improving the overall aircraft fuel economy over multiple flight missions and conditions in practical scenarios. The proposed optimal design approach starts with the flight route data analysis to cluster the flight operational data using gaussian mixture model, so that a concise representation of flight mission profiles can be achieved. Then, an optimal orthogonal array-based Latin hypercubes are employed to generate sampling points of design variables for electrified aircraft propulsion. The mission analysis is performed with coupled propulsion-airframe integration in order to propose energy management strategy for mission-dependent aircraft performance. Consequently, fuel economy surrogate model is established via support vector machines to obtain the optimal design points of electrified aircraft propulsion. For assessing the viability of novel propulsion technologies, techno-economic evaluation is conducted using sensitivity analysis and breakeven electricity prices under a series of environmental regulatory policy scenarios.

Preliminary Sizing Method for Hybrid-Electric Distributed-Propulsion Aircraft

The use of hybrid-electric propulsion (HEP) entails several potential benefits such as the distribution of power along the airframe, which enables synergistic configurations with improved aerodynamic and propulsive efficiency. This paper presents a comprehensive preliminary sizing method suitable for the conceptual design process of hybrid-electric aircraft, taking into account the powertrain architecture and associated propulsion–airframe integration effects. To this end, the flight-performance equations are modified to account for aeropropulsive interaction. A series of component-oriented constraint diagrams are used to provide a visual representation of the design space. A HEP-compatible mission analysis and weight estimation are then carried out to compute the wing area, powerplant size, and takeoff weight. The resulting method is applicable to a wide range of electric and hybrid-electric aircraft configurations and can be used to estimate the optimal power-control profiles. For demonstration purposes, the method is applied to a regional HEP aircraft featuring leading-edge distributed propulsion (DP). Three powertrain architectures are compared, showing how the aeropropulsive effects are included in the model. Results indicate that DP significantly increases wing loading and improves the cruise lift-to-drag ratio by 6%, although the growth in aircraft weight leads to an energy consumption increase of 3% for the considered mission.

Large electric machines for aircraft electric propulsion

To achieve benefits similar to those seen in hybrid-/all-electric ground-based and marine vehicles, electric propulsion has been proposed for large commercial aircraft. Among the main drivers of this are improved fuel economy, reduced harmful emissions, and lower audible noise. In converting to electric propulsion, the added electrical components' masses must be minimised so that the benefits that the components enable - improved turbine efficiency, distributed propulsion and propulsion-airframe integration - are not cancelled out by their weight penalty. This puts stringent requirements on the large electric machines used in the system, both those that generate electric power from the turbine shaft and those that drive propellers or ducted fans, because they are among the heaviest of the added electric components. A key machine design metric in this application is the specific power (SP), or the power-to-mass ratio. This study gives a comprehensive overview of large electric machines for aircraft electric propulsion applications, with a focus on methods for mass reduction and SP improvement.

Wavepacket modeling of the jet noise source

This study is motivated by the need for physical models for the jet noise source to be used in practical noise prediction schemes for propulsion–airframe integration concepts. The basis for the source model is an amplitude-modulated traveling wave—the wavepacket. The source is parameterized and the parameters are determined by minimizing the difference between the modeled and experimental sound intensity distributions in the far field. Even though the pressure signal that reaches the far field is highly filtered, sufficient information is available to construct a wavepacket with reasonable physical characteristics. A simple stochastic extension of this concept shows a connection between the shape of the far-field sound pressure level spectrum and the emission polar angle. It suggests that the broadening of the spectrum with increasing polar angle from the downstream axis can be explained on the basis of a single noise source (the wavepacket), rather than the prevailing model of two distinct noise sources, one coherent and the other incoherent.

Benefit evaluation of hybrid electric propulsion concepts for CS-23 aircraft

The importance of small passenger aircraft design fitting up to nine passengers and the evaluation of associated technologies have been recently increasing. This is related to the upcoming interest in on demand air mobility concepts not only for intra- or inter-urban transport but also for thin-haul routes ranging from 150 km to approximately 500–800 km. Such concepts seem feasible as key enabling technologies (e.g., flight automation and battery technology) are likely to be mature enough to enable these concepts in the foreseeable future. However, until battery specific energy in particular surpasses the threshold of approximately 400 Wh/kg, hybrid electric propulsion concepts could serve as an interim solution. Therefore, this paper deals with the question of how hybrid electric concepts score compared to conventional piston engine aircraft and which concept promises the most benefits. This includes consideration of propulsion-airframe integration benefits of electric engines through distributed electric propulsion (DEP). Results show that the series hybrid electric concept is superior to a parallel setup if at least a 15% higher cruise lift-to-drag ratio can be achieved due to DEP (30–50% increase likely). Despite higher weight, variable operating costs can be reduced by 15–35% with application of series hybrid electric propulsion concepts.

Propulsion-Airframe Integration Using Statistical Surrogates from Computer Simulations

Propulsion-Airframe Integration Using Statistical Surrogates from Computer Simulations

Boundary-Layer Transition on X-43A

The successful Mach 7 and 10 flights of the first fully integrated scramjet propulsion systems by the Hyper-X (X-43A) program have provided the means with which to verify the original design methodologies and assumptions. As part of Hyper-X s propulsion-airframe integration, the forebody was designed to include a spanwise array of vortex generators to promote boundary layer transition ahead of the engine. Turbulence at the inlet is thought to provide the most reliable engine design and allows direct scaling of flight results to groundbased data. Pre-flight estimations of boundary layer transition, for both Mach 7 and 10 flight conditions, suggested that forebody boundary layer trips were required to ensure fully turbulent conditions upstream of the inlet. This paper presents the results of an analysis of the thermocouple measurements used to infer the dynamics of the transition process during the trajectories for both flights, on both the lower surface (to assess trip performance) and the upper surface (to assess natural transition). The approach used in the analysis of the thermocouple data is outlined, along with a discussion of the calculated local flow properties that correspond to the transition events as identified in the flight data. The present analysis has confirmed that the boundary layer trips performed as expected for both flights, providing turbulent flow ahead of the inlet during critical portions of the trajectory, while the upper surface was laminar as predicted by the pre-flight analysis.

Installation Effects on the Flow and Noise of Wing Mounted Jets

T IS known experimentally, since the late 1970s, that a jet installed under a wing of an aircraft radiates more noise than the same jet in a standalone condition. The excess noise is the propulsion-airframe integration noise or commonly referred to as installation noise. When a jet is placed near a wing, there is an increase in noise in the flyover directions because of the reflection of sound by the wing. Here, installation noise includes not merely the noise increase due to the reflection of sound by the wing. The major part of this noise is generated aerodynamicly by the nonlinear interaction between the flow around the wing flap and the jet. In this work,ourprimaryinterestistomodelandtopredictinstallationnoise of aerodynamic origin. Installation noise increases not only the total aircraftnoiseinthe flyoverplanebutalsointhesidelinedirections.It is especially important during landings and takeoffs when the flaps are down. Duringthe1980s,anumberofexperimentswerecarriedouttrying to quantify the characteristics and intensity of installation noise [1– 4]. Most of these experiments involved the measurements of the jet alone noise and the noise when the jet was placed near a model of an aircraft wing inside an anechoic chamber. The experimental measurements by Wang [2] were the most systematic. In his experiment,ascaledmodelofthewingofaDC-10aircraftwasused. Large noise increase was observed in the flyover plane in the lowfrequencypartofthespectrum.Theincreaseinhigh-frequencynoise was less. In directions at small exhaust angles, the installation noise intensity was quite low. In the sideline, the radiated noise characteristics, on the other hand, were quite different. Overall, the measured data indicated that installation noise had a unique spectral shape and a directional pattern of its own. Recently, there is a renewed interest in propulsion-airframe integration noise. Mead and Strange [5] investigated the under-thewing installation effects on jet noise with special emphasis on the sidelinedirections.Theirinterestinthesidelinewasmotivatedbythe experience that it was generally more difficult to meet legislative limit on sideline noise level requirements. They reported the measurement of high installation noise level in the low-frequency range. One drawback of the Mead and Strange experiment [5], as well as most of the previous works, is that the experiments were carried out in static conditions. Upon realizing that the effect of forward flight is extremely important in the interaction between the flow around the wing flapandthejet,aseriesofnewexperimentsoninstallationnoise was conducted by engineers of The Boeing Company (Shivashankara and Blackner [6], Blackner and Bhat [7], and Bhat and Blackner [8]). They employed an open wind tunnel at M � 0:28 tosimulatetheforwardmotionoftheaircraft.Byusingellipticmirror microphones and the newly developed phase-array microphones, they were able to obtain noise source location maps as well as farfield noise data.

NASA’s propulsion airframe aeroacoustics research

The integration of propulsion and airframe is a fundamental consideration in the design of an aircraft system. Many considerations influence the integration, such as structural, aerodynamic, and maintenance factors. In the future, a focus on the aerodynamic and acoustic interaction effects of installation, propulsion airframe aeroacoustics will become more important as noise reduction targets become more difficult to achieve. In addition to continued fundamental component reduction efforts, a system level approach that includes propulsion airframe aeroacoustics will be required in order to achieve the 20-dB noise reductions envisioned by the aggressive NASA goals. This emphasis on the aeroacoustics of propulsion airframe integration is a new part of NASA’s ongoing acoustics research. The presentation will review current efforts and highlight technical challenges and approaches.

NASA hypersonic flight demonstrators—overview, status, and future plans

NASA's Next Generation Launch Technology (NGLT) program is developing and maturing advanced propulsion and vehicle systems technologies and flight vehicle concepts to enable future development of safer and more economical launch systems. Within NGLT, NASA is developing advanced air breathing propulsion systems and demonstrating these systems in hypersonic flight vehicles. The flight demonstrations are necessary to fully validate these technologies for application to future space launch vehicles and other flight systems. NASA's Hyper-X Program (X-43A) began the effort to flight demonstrate hypersonic air breathing propulsion systems to provide technologies that will enable development of safer and more economic space access vehicles in the future. Following X-43A, NASA, in collaboration with the United States (US) Department of Defense (DoD), is developing additional, progressively more complex hypersonic X-vehicles that will demonstrate new air breathing propulsion systems, propulsion-airframe integration, and other vehicle systems technologies required for high speed flight up to Mach 15. These technologies will contribute to safer, more reliable and more economic future launch systems and hypersonic aircraft/missiles. This paper describes NASA's current hypersonic flight demonstration projects, status of the efforts, and plans for future vehicles.

Propulsion airframe integration design, analysis and challenges going into the 21st century

AbstractPropulsion airframe integration design and analysis has many challenges as we move into the 21st century. Along with the conventional challenges of integrating a propulsion system on an aircraft, new technologies and new business drivers dictate innovative approaches to the design process be developed and applied.On the technical side, new innovations in jet noise suppression and new aircraft concepts such as the blended wing body (BWB) will give rise to many challenges in propulsion system performance, operability, and meeting system requirements such as thrust reverse capability. Aircraft engine and aircraft manufacturers must have the appropriate design and analysis tools in place which provide the ability to react quickly to inevitable design changes, driven by constantly changing requirements, during the product development cycle.On the business side, the rapid globalisation of the business dictates that the latest electronic technology be utilised to enable speed in communication with global customers as well as revenue sharing partners. More than ever, cost and schedules dictate the use of analytical methods to minimise the amount of qualification testing. Design and analysis software must be flexible and capable of integrating CAD/CAM and CAE tools while maintaining configuration control of the product.The following paper describes some of the new technical challenges facing the industry. Innovative methods of addressing those challenges are described.

Air-Breathing Hypersonic Cruise: Prospects for Mach 4–7 Waverider Aircraft

There is currently a renewal of world-wide interest in hypersonic flight. Vehicle concepts being considered range from cruise missiles to SSTO and TSTO vehicles. The new characteristics of these vehicles are that they will be powered by air-breathing engines and have long residence times in the air-breathing corridor. In the Mach 4–7 regime, waverider aircraft are being considered as candidates for both long-range and short-range cruise missions, as hypersonic missiles, and as high-L/D highly maneuverable vehicles. This paper will discuss the potential for near-term and far-term application of air-breathing engines to the above-mentioned waverider vehicle concepts and missions. In particular, the cruise mission is discussed in detail and attempts are made to compare and contrast it with the accelerator mission. Past criticisms levied against waveriders alleging low volumetric efficiency, lack of engine/airframe integration studies, poor off-design performance, poor take-off and landing capability, have been shown by ongoing research to be unfounded. A discussion is presented of some of the technical challenges and ongoing research aimed at realizing such vehicles: from turboramjet and scramjet technology development, propulsion-airframe integration effects on vehicle performance, aeroservothermoelastic systems analysis, hypersonic stability and control with aeroservothermoelastic and propulsion effects, etc. A unique and very strong aspect of hypersonic vehicle design is the integration and interaction of the propulsion system, aerodynamics, aerodynamic heating, stability and control, and materials and structures. This first-order multidisciplinary situation demands the ability to integrate highly coupled and interacting elements in a fundamental and optimal fashion to achieve the desired performance. Some crucial technology needs are found in propulsion-airframe integration and its role in configuration definition, hypersonic boundary-layer transition and its impact on vehicle gross-weight and mission success, scramjet combustor mixing length and its impact on engine weight and, CFD (turbulence modeling, transition modeling, etc) as a principal tool for the design of hypersonic vehicles. Key technology implications in thermal management, structures, materials, and flight control systems will also be briefly discussed. It is concluded that most of the technology requirements in the Mach 4–7 regime are relatively conventional, making cited applications near-term, yet offering very significant advancements in aircraft technology.

Hypersonic flight experience

The flight test results from the X-15, Asset, Prime, Reentry F and Shuttle Orbiter flight research programmes are reviewed and compared with theory and ground- based experiments. Primary emphasis is placed on our present capability to predict aerodynamic coefficients and stability derivatives, and distributions of surface pressure and aerodynamic heating rate for typical orbital and sub-orbital hypersonic vehicles. Overall, this comparison demonstrates the feasibility of designing hypersonic vehicles based on tests in conventional perfect-gas wind tunnels supplemented by state-of-the-art CFD analysis. At Mach numbers up to approximately 8, real-gas effects are small and Mach number/Reynolds number simulation is sufficient to insure accurate prediction of aerodynamic characteristics. At higher mach numbers, real-gas effects become important but appear to affect primarily pitching moment and to have little influence on other aerodynamic characteristics. Viscous interaction effects appear to be well correlated by the viscous interaction parameter V̄' ∞ , and to affect primarily axial force. With the exception of RCS jet interactions, stability and control derivatives are well predicted throughout the hypersonic flight régime. State-of-the-art aerodynamic heating techniques appear to give accurate predictions for laminar and fully turbulent attached flows so long as there are no strong shock interactions or non-equilibrium chemistry effects. For such flows, heating rate distributions depend only weakly on Mach number for M ∞ ≥ 8 and hence M ∞ ≈ 8-10 wind tunnel results can be used throughout the hypersonic speed range. For high- speed, high-altitude flight, surface catalycity effects can have a major influence on heating rates but modern, finite-rate boundary layer analyses are capable of predicting major trends. The major remaining challenges are the accurate prediction of: (1) real-gas effects on longitudinal trim, (2) the effectiveness of blended high altitude control systems, (3) shock interaction heating, (4) heating rates for separated vortex-dominated leeside flows, and (5) boundary layer transition and relaminarzation. Finally, it is pointed out that there are no flight data on the propulsion-airframe integration effects that are so important for airbreathing launch vehicles.